Electric motors are used for various technical applications. In electric or hybrid vehicles, electric motors are, for example, used for driving the motor vehicle. For energization of electric motors, it is necessary to detect the rotational speed or rotation angle of a rotor of the electric motor. For this purpose, the electric motor is equipped with an appropriate sensor for detecting the rotation angle. The angular velocity and/or the angular acceleration of the rotor may be calculated from the detected rotation angle. Resolvers or digital sensors are used as sensors for detecting the rotation angle. Resolvers are analog measuring devices which induce a voltage which is proportional to the rotation angle of the rotor or the electric motor. In this way, the rotation angle and the angular velocity may be detected very accurately, in particular at low rotational speeds, for example less than 1000 RPM. However, resolvers are susceptible to errors at high rotational speeds, since the evaluation of the analog voltage induced by the rotational motion may become inaccurate. In contrast to that, digital sensors, also known as incremental encoders, output a digital signal which, together with a time function, may be converted into a rotational speed signal. Digital sensors distinguish themselves by a great robustness and are thus particularly well suited for use in electric or hybrid vehicles. However, digital sensors have the disadvantage that in the case of low rotational speeds the number of the measured increments per time unit may be low and therefore only a low degree of accuracy of the detected rotation angle is achieved at low rotational speeds. A high degree of accuracy of the rotation angle measured by the digital sensor is, however, necessary to be able to set the torque for the energization of the electric motor.
A method for operating a speed-variable electric motor, in particular a brushless servomotor, is described in German Patent No. DE 41 22 391 A1. By regulating the motor variables current, position angle and rotational speed, the rotational speed is determined through a filter from the position angle measured with the aid of a position sensor.